Hitherto, a technology disclosed in Patent Literature 1 has been known as a variable compression ratio mechanism. According to Patent Literature 1, a multi-link type piston and crank mechanism are used to change stroke characteristics of a piston so that a mechanical compression ratio of an internal combustion engine can be changed. Specifically, the piston and a crankshaft are coupled to each other through intermediation of an upper link and a lower link. A posture of the lower link is controlled by turning of a control shaft connected to an actuator including a drive motor, and a wave gear type speed reducer. With this configuration, the stroke characteristics of the piston are changed to control the engine compression ratio. Further, bearings are provided on both sides of the wave gear type speed reducer in an axial direction, and with those bearings, the wave gear type speed reducer is rotatably supported.